Calculator Inputs
Enter your project assumptions below. The result block stays above this form after calculation, exactly below the header section.
Formula Used
q = p × s
where q is line load, p is design pressure, and s is mullion tributary width.
M = qL2 / Cm
Cm = 8 for simply supported, 12 for fixed-fixed, and 2 for cantilever.
V = qL / Cv
Cv = 2 for simply supported or fixed-fixed, and 1 for cantilever.
Zreq = M / σallow
This checks bending stress against allowable material stress.
δ = Cd × qL4 / (EI)
Cd = 5/384 for simply supported, 1/384 for fixed-fixed, and 1/8 for cantilever.
δallow = L / limit ratio
For example, L/175 means the span divided by 175.
How to Use This Calculator
- Enter facade width and height to define the total glazed area.
- Set your target mullion spacing and target support span between anchors.
- Enter design wind pressure and any chosen load factor.
- Select the support condition that best represents the mullion restraint.
- Enter material stiffness, allowable stress, and the deflection limit ratio.
- Add the section modulus and inertia of the proposed mullion profile.
- Optionally enter unit weight for quantity and approximate weight reporting.
- Press the calculate button. Review pass status, required properties, graph, and export files.
Example Data Table
| Project | Facade Size | Spacing | Span | Wind Pressure | Required Z | Required I | Status |
|---|---|---|---|---|---|---|---|
| Office Tower Podium | 18 m × 12 m | 1500 mm | 3000 mm | 1.50 kPa | 59.33 cm³ | 432.14 cm⁴ | Pass with Z=75, I=510 |
| Retail Front Elevation | 12 m × 8 m | 1200 mm | 2800 mm | 1.20 kPa | 36.54 cm³ | 214.37 cm⁴ | Pass with Z=45, I=260 |
| Airport Curtain Wall | 30 m × 20 m | 1600 mm | 3200 mm | 2.10 kPa | 107.52 cm³ | 821.90 cm⁴ | Review if section is smaller |
Frequently Asked Questions
1) What does this calculator actually check?
It estimates mullion demand from wind pressure, then compares required section modulus and required inertia against the profile you entered. It also reports moment, shear, stress, deflection, geometry counts, total length, and approximate weight for quick preliminary design review.
2) Is the result suitable for final construction approval?
No. It is best for concept design, budgeting, and early engineering comparison. Final approval should include code-specific combinations, connection stiffness, bracket slip, glass bite, seismic movement, thermal actions, and manufacturer test data or formal structural calculations.
3) Why does the calculator show actual spacing and actual support span?
The tool converts your target spacing and target support span into whole bay counts and whole support segments. That creates actual dimensions that fit the full facade size. Using those actual values gives a more realistic preliminary design check.
4) Which support condition should I choose?
Choose the option closest to real restraint. Simply supported suits pinned behavior, fixed-fixed suits stronger end restraint, and cantilever suits a free top or bottom condition. If you are unsure, use the more conservative assumption and confirm with the facade engineer.
5) What units does the calculator use internally?
Geometry is converted to millimeters, pressure is converted from kilopascals to newtons per square millimeter, modulus and stress are in megapascals, section modulus is in cubic centimeters input, and inertia is in fourth-power centimeters input.
6) Why can a mullion fail deflection while passing stress?
Stress depends on section modulus, while serviceability depends strongly on moment of inertia and span. A profile can be strong enough in bending but still feel too flexible. Curtain wall systems often become deflection-controlled before they become stress-controlled.
7) Can I use steel values instead of aluminum?
Yes. Replace the modulus of elasticity and allowable stress with appropriate project values for the material and code basis you are using. The method still works, but the chosen section properties must match the exact profile shape and axis.
8) What should I do if coverage is under 100%?
Increase section properties, shorten support span, reduce spacing, improve support restraint, or reassess the design pressure basis. Any of those changes can lower demand or raise capacity. Recalculate and compare the updated utilization graph and result cards.